Include primitives.css in the main primer.css export (#2657)

Author: jonrohan

.changeset/little-toes-reflect.md

@@ -0,0 +1,5 @@
+---
+"@primer/css": patch
+---
+
+Import `primitives/index.scss` in the main `primer.css` file.

src/index.scss

@@ -12,6 +12,7 @@
 
 // CSS color variables
 @import './color-modes/index.scss';
+@import './primitives/index.scss';
 
 // Global requirements
 @import './core/index.scss';